Adults who complete the diphtheria tetanus pertussis (DTP) vaccine series during childhood should receive a tetanus-diphtheria toxoid (Td) booster vaccine every 10 years. We studied awareness of and willingness to receive the Td booster vaccine as well as factors influencing these attitudes. A total of 1,284 participants were interviewed using a questionnaire at the out-patient department (OPD) of Siriraj Hospital, Bangkok, Thailand from January 31 to February 28, 2011. The sample population included: OPD patients, family members accompanying patients and medical personnel other than doctors. Seven percent of the 961 general participants and 26.3% of the 320 medical personnel who completed the questionnaire were aware of the need to receive the Td booster vaccine. Eighty-six point five percent of participants stated they be willing to receive the vaccine if they were instructed by their doctors. Logistic regression analyses showed the following factors correlated with awareness of the need to receive the vaccine: born after 1978 (aOR = 2.17; 95% CI: 1.59-2.97), higher income (aOR = 1.93; 95% CI: 1.37-2.72), and higher general preventive awareness score (aOR = 1.08; 95% CI: 1.02-1.14). Factors that correlated with willingness to receive the vaccine were: higher education (aOR = 2.17; 95% CI: 1.54-3.05), a higher score on general knowledge of tetanus-diphtheria (aOR = 2.00; 95% CI: 1.15-3.51), a higher general preventive awareness score (aOR = 1.06; 95% CI: 1.007-1.115), and previous experience with the Td booster vaccine (aOR = 2.85; 95% CI: 1.71-4.76), Under-awareness of the adult vaccination program remains a problem in Thailand especially among older generations and low-socioeconomic groups. As passive immunity wanes with time, public health authorities need to take action to educate the general population and health care workers about the importance of the Td booster vaccine.